Electrophotographic photosensitive bodies recently proposed and utilized are as follows: a laminated organic electrophotographic photosensitive body (OPC) in which a photosensitive layer has at least two layers, that is, a charge generating layer (CGL) that generates charge by exposure and a charge transporting layer (CTL) that transports charge, and a monolayer organic electrophotographic photosensitive body in which a photosensitive layer is composed of a single layer obtained by dispersing a charge generating substance and a charge transporting substance in a binder resin or by dispersing only a charge generating substance in a binder resin.
Further, both the laminated and monolayer electrophotographic photosensitive bodies each provided with a protective layer (OCL) for the protection of its surface layer have been utilized in view of a problem to be described later.
An organic electrophotographic photosensitive body is requested to have predetermined sensitivity, predetermined electrical characteristics, and predetermined optical characteristics in accordance with an electrophotographic process to be applied.
Electrical and mechanical external forces are applied to the surface of the photosensitive layer of the electrophotographic photosensitive body every time an operation such as corona charging or contact charging, development with toner, the transfer of toner onto paper, or a cleaning treatment is performed because the surface is repeatedly subjected to such operation.
Therefore, the photosensitive layer provided to the surface of the electrophotographic photosensitive body is requested to have durability against those external forces in order that the image quality of an electrophotograph may be maintained for a long time period.
To be specific, the photosensitive layer is requested to have durability against: the generation of wear or a flaw on its surface due to friction; and the deterioration of its surface due to an active gas such as ozone or discharge in corona charging, contact charging, or transfer.
A polycarbonate resin using, for example, 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) or 1,1-bis(4-hydroxyphenyl)cyclohexane (bisphenol Z) having good compatibility with a charge transporting substance for use in a photosensitive layer and good optical characteristics as a starting material has been heretofore used as a binder resin for an organic electrophotographic photosensitive body to respond to such requests.
However, even such polycarbonate resin using bisphenol A or bisphenol Z as a raw material does not sufficiently satisfy the above requests, and a large number of methods each involving the use of a polycarbonate resin or any other resin having a structure except bisphenol A and bisphenol Z have been proposed and put into practical use.
In recent years, the surface of a photosensitive body is requested to have low surface energy, in particular, to maintain low surface energy in order that high cleaning property may be realized in association with the fact that a printing machine or copying machine employing an electrophotographic process has become possible to represent colors.
For example, an approach involving dispersing an additive for imparting hydrophobicity or fine particles each made of a material having low surface energy has been taken to respond to the above-mentioned request. However, the additive is apt to exude (bleed out) from an electrophotographic photosensitive body, and the fine particles each made of a material having low surface energy are apt to agglomerate, so the additive and the fine particles involve problems such as light scattering in the photosensitive body and insufficient dispersibility at the time of the production of the photosensitive body.
In addition, attempts such as the change of a binder resin and the addition of various additives have been made to improve the dispersibility of each of various fine particles (Patent Documents 1 and 2). However, each of the change of a binder resin and the addition of various components leads to the deterioration of the electrophotographic characteristics of an electrophotographic photosensitive body such as a reduction in sensitivity of the body, thereby causing another problem.
Patent Document 1: JP 63-65451 A
Patent Document 2: JP 05-45920 A